XO sex-determination system

Heredity of sex chromosomes in XO sex determination

The XO sex-determination system (sometimes referred to as X0 sex-determination system) is a system that some species of insects, arachnids, and mammals use to determine the sex of offspring. In this system, there is only one sex chromosome, referred to as X. Males only have one X chromosome (XO), while females have two (XX). The letter O (sometimes a zero) signifies the lack of a Y chromosome.^[1] Maternal gametes always contain an X chromosome, so the sex of the animals' offspring depends on whether a sex chromosome is present in the male gamete. Its sperm normally contains either one X chromosome or no sex chromosomes at all.

This system determines the sex of offspring among:

• Most arachnids^[2] with the exception of mites where a small majority are haplodiploid,^[3]
• Almost all apterygote and Paleopteran insects (e.g., dragonflies, silverfish)
• Most exopterygote insects (e.g., grasshoppers, crickets, cockroaches)
• Some nematodes,^[2] crustaceans,^[2] gastropod molluscs,^[4] and bony fish,^[5] notably in the genus Ancistrus^[6]
• Several mammals, including:
  • A few species of bat, including the hammer-headed bat,^[7] Buettikofer's epauletted fruit bat, Franquet's epauletted fruit bat, Peters's epauletted fruit bat, and Gambian epauletted fruit bat^[8]
  • The Ryukyu spiny rat and Tokunoshima spiny rat^[9]

In a variant of this system, most individuals have two sex chromosomes (XX) and are hermaphroditic, producing both eggs and sperm with which they can fertilize themselves, while rare individuals are male and have only one sex chromosome (XO). The model organism Caenorhabditis elegans—a nematode frequently used in biological research—is one such organism.

Most spiders have a variation of the XO system in which males have two different X chromosomes (X₁X₂O), while females have a pair of X₁ chromosomes and a pair of X₂ chromosomes (X₁X₁X₂X₂).^[1] Some spiders have more complex systems involving as many as 13 different X chromosomes.^[1]

Some Drosophila species have XO males.^[10] These are thought to arise via the loss of the Y chromosome.^{[original research]}

In humans the XO designation attaches to individuals with Turner syndrome.

Evolution

XO sex determination can evolve from XY sex determination within about 2 million years.^{[clarification needed]} It typically evolves due to Y-chromosome degeneration. As the Y-chromosome is not paired (though see pseudoautosomal region), it is susceptible to decay by Muller's ratchet.^[11]

Similarly, the W chromosome in a ZW sex-determination system is susceptible to decay, resulting in a ZZ/ZO system.

Parthenogenesis

Parthenogenesis with XO sex-determination can occur by different mechanisms to produce either male or female offspring.^[12]

See also

• Sex-determination system
• Sexual differentiation
• Haplodiploid sex-determination system
• XY sex-determination system
• ZO sex-determination system
• ZW sex-determination system
• Temperature-dependent sex determination
• X chromosome
• Y chromosome

References

1. Sember, Alexandr; Pappová, Michaela; Forman, Martin; Nguyen, Petr; Marec, František; Dalíková, Martina; Divišová, Klára; Doležálková-Kaštánková, Marie; Zrzavá, Magda; Sadílek, David; Hrubá, Barbora; Král, Jiří (24 July 2020). "Patterns of Sex Chromosome Differentiation in Spiders: Insights from Comparative Genomic Hybridisation". Genes. 11 (8): 849. doi:10.3390/genes11080849. PMC 7466014. PMID 32722348.
2. Bull, James J.; Evolution of sex determining mechanisms; p. 17 ISBN 0805304002
3. Bachtrog, Doris; Mank, Judith E.; Peichel, Catherine L.; Kirkpatrick, Mark; Otto, Sarah P.; Ashman, Tia-Lynn; Hahn, Matthew W.; Kitano, Jun; Mayrose, Itay; Ming, Ray; Perrin, Nicolas; Ross, Laura; Valenzuela, Nicole; Vamosi, Jana C. and The Tree of Sex Consortium; ‘Sex Determination: Why So Many Ways of Doing It?’; PLoS Biol12(7): e1001899
4. Thirot-Quiévreux, Catherine (2003). "Advances in Chromosomal Studies of Gastropod Molluscs". Journal of Molluscan Studies. 69 (3): 187–201. doi:10.1093/mollus/69.3.187.
5. Devlin, R.H. and Y. Nagahama, 2002. ‘Sex determination and sex differentiation in fish: an overview of genetic, physiological, and environmental influences’; Aquaculture 208: 191–364.
6. Anderson, Luís Alves; Oliveira, Claudio; Nirchio, Mauro; Granado, Ángel and Foresti, Fausto; ‘Karyotypic relationships among the tribes of Hypostominae (Siluriformes: Loricariidae) with description of XO sex chromosome system in a Neotropical fish species’; Genetica, vol. 128 (2006); pp. 1-9
7. Hsu, T. C.; Benirschke, Kurt (1977). "Hypsignathus monstrosus (Hammer-headed fruit bat)". An Atlas of Mammalian Chromosomes. pp. 13–16. doi:10.1007/978-1-4615-6436-2_4. ISBN 978-1-4684-7997-3.
8. Denys, C.; Kadjo, B.; Missoup, A. D.; Monadjem, A.; Aniskine, V. (2013). "New records of bats (Mammalia: Chiroptera) and karyotypes from Guinean Mount Nimba (West Africa)". Italian Journal of Zoology. 80 (2): 279–290. doi:10.1080/11250003.2013.775367. hdl:2263/42399. S2CID 55842692.
9. Kobayashi, Tsuyoshi; Yamada, Fumio; Hashimoto, Takuma; Abe, Shintaro; Matsuda, Yoichi; Kuroiwa, Asato (2007). "Exceptional minute sex-specific region in the XO mammal, Ryukyu spiny rat". Chromosome Research. 15 (2): 175–187. doi:10.1007/s10577-006-1093-y. PMID 17294259. S2CID 6461447.
10. Patterson, J. T.; Stone, W. S. (1952). Evolution in the Genus Drosophila. New York: Macmillan.
11. Nei, Masatoshi (2013-05-02). Mutation-Driven Evolution. OUP Oxford. p. 168. ISBN 978-0-19-163781-0.
12. Hales, Dinah F.; Alex C. C. Wilson; Mathew A. Sloane; Jean-Christophe Simon; Jean-François Legallic; Paul Sunnucks (2002). "Lack of Detectable Genetic Recombination on the X Chromosome During the Parthenogenetic Production of Female and Male Aphids". Genetics Research. 79 (3): 203–209. doi:10.1017/S0016672302005657. PMID 12220127.